1. Field of the Invention
The present invention generally relates to the on-line detection of characteristics of sheet materials and, more particularly, to spectrometric methods for on-line detection of characteristics of sheet materials.
2. State of the Art
It is often desirable to obtain measurements of selected characteristics of sheet materials during manufacture. Although various properties of sheet materials can be detected by off-line laboratory testing, such tests often are not practical because of the time required for sample acquisition and analysis. Also, laboratory testing has the shortcoming that samples obtained for testing may not accurately represent sheet material that has been produced.
To overcome the drawbacks of laboratory testing of sheet materials, various sensor systems have been used for detecting sheet properties "on-line," i.e., on a sheet-making machine while it is operating. Typically, on-line sensor devices are operated to periodically traverse, or "scan," traveling webs of sheet material during manufacture. Scanning usually is done in the cross direction; i.e., in the direction perpendicular to the direction of sheet travel. Depending upon the sheet making operation, cross-directional distances can range up to about 400 inches or more. Typically, the rate of travel of a sheet through a scanning sensor system ranges from about forty to about four-hundred feet per minute.
Although a wide variety of scanning sensor devices have been used for on-line measurements of sheet materials, there have been difficulties in attempts to use on-line spectrometers based upon interferometers for such purposes. In part, the difficulties arise because interferometers normally are not designed for use in manufacturing environments where the instruments may be subjected to substantial vibrations from heavy and high-speed rotating machinery. Also, difficulties arise in using interferometers in on-line applications because on-line scanning sensors usually only view a sample area of a traveling sheet only for a limited time, whereas laboratory-type interferometers normally can analyze a fixed-position sample for as long as necessary to obtain a complete spectral display. Still other difficulties in applying conventional (e.g., laboratory) interferometers to on-line applications arise because the laboratory instruments are normally bulky.